Effects of monomer structure and imidization degree on mechanical properties and viscoelastic behavior of thermoplastic polyimide films

EUROPEAN POLYMER JOURNAL

European Polymer Journal 42 (2006) 1844–1854

www.elsevier.com/locate/europolj

Effects of monomer structure and imidization degree on mechanical properties and viscoelastic behavior of thermoplastic polyimide films M.B. Saeed, Mao-Sheng Zhan

*

School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 37, Xue Yuan Road, Beijing 100083, PR China Received 4 October 2005; received in revised form 25 February 2006; accepted 5 March 2006 Available online 19 April 2006

Abstract Two thermoplastic polyimides based on a common diamine (3,4 0 -ODA) were synthesized using different dianhydrides, namely ODPA and BPDA by a two step method. Molecular weight was controlled by using PA as an end capping agent. Effects of imidization degree on the mechanical properties and viscoelastic behavior of thermoplastic polyimide films were investigated. Film samples with varying degrees of imidization were characterized using FTIR, DMTA and tensile properties testing. It was found that two polyimides have different rates of imidization because of difference in monomer reactivity and molecular structure. It was observed that with an increase in imidization degree there was a decrease in thermoplastic response and a change in viscoelastic behavior from liquid-like to solid-like. With increase in imidization degree the tensile modulus and tensile strength of the films were increased, whereas elongation at break and tensile breaking energy were found to decrease after a certain imidization temperature.  2006 Elsevier Ltd. All rights reserved. Keywords: Thermoplastic polyimide; Degree of imidization; FTIR; Tensile strength testing; Viscoelastic behavior

1. Introduction Aromatic polyimides (PI) have been utilized in industrial applications such as automobile and aircraft parts, electronic packaging, adhesives, and matrix materials for composite materials due to their excellent mechanical, thermal, dielectric and adhe-

*

Corresponding author. Tel.: +86 10 823 17120; fax: +86 10 823 15666. E-mail addresses: [email protected] (M.B. Saeed), [email protected] (M.-S. Zhan).

sive properties, chemical resistance and dimensional stability [1]. Thermoplastic PIs are commonly synthesized in form of films for various industrial applications because of ease in processing, handling and storage [2,3]. The most common method for PI film casting is solvent-cast free-standing films from poly(amic acid) (PAA) precursor. This method of PI synthesis consists of two steps; the solution condensation of an aromatic diamine and a dianhydride in a polar aprotic solvent such as NMP to form PAA which could be cast into films, followed by thermal cyclodehydration of the amide-acid to form PI [4].

0014-3057/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.03.004

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

Properties of thermoplastic PI films with intended use as structural adhesives or laminations for composite plies have been the subject of many studies in the past. These specific applications require thermoplastic PIs to exhibit enhanced melt flow behavior at relatively low processing temperatures and pressures to achieve proper wetting of adherends or that of fibers. Nobuyuki et al. [5] have tested PI films imidized at 300 C without molecular weight control for adhesive properties with steel adherends. They used a pressure of 19.6 MPa for the bonding of joints at a temperature 50 C above Tg to achieve adhesive strengths comparable with other polymeric adhesives. Use of high pressures in the range of 20 MPa could result in various problems such as flow out of resin, damage to bonded structure and entrapment of volatile products thus causing voids. Hou et al. [6] investigated carbon fiber and thermoplastic PI composite properties. They achieved good wetting of fiber layers by lowering the molecular weight of the PI to a range of 14–15 kg mole1 and used only 2 MPa consolidation pressure. Nondestructive testing of the plies confirmed absence of any voids or cavities in the composite plies. However by lowering the molecular weight mechanical properties and thermal stability of the polyimide was adversely affected. Some researchers have tried other methods to enhance thermoplastic PIs melt flow behavior. Some of the methods tried are, use of highly flexible monomer units like BAPS, BAPSM, TPER, etc. [5], addition of reactive diluents [7], forming copolyimides with flexible thermoplastics like diaminopolysiloxane [8] and lowering of molecular weights by end capping technology [9–11]. All these methods though are successful in enhancing the melt flow at low processing temperatures and pressures but resulting polyimides have very low mechanical properties and poor thermal stability. In order to benefit from these researches it is desirable to use a combination of all these methods in such a manner that effect on polyimide properties is minimal. In this study a new approach has been suggested to achieve enhanced melt flow behavior of thermoplastic PIs at processing temperature by investigating the feasibility of using partially imidized PI films. In addition to partial imidization further improvement in melt flow behavior is achieved by utilizing a combination of semi-rigid monomers and by controlling the molecular weight to a somewhat higher value as compared to previous researches. By achieving a balance between degree of imidization, monomer flexi-

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bility and PI molecular weight it is expected to achieve enhanced melt flow at low processing temperature and pressure without sacrificing polyimide properties. Degree of imidization of PI is sensitive to imidization temperature and increases with an increase in imidization temperature. A PI with a higher degree of imidization has stiff molecular chains and thus lacks fluidity. Even at temperatures approaching glass transition temperature, the stiff molecular chains exhibit limited mobility unless they are excessively short in length or have highly flexible molecular structure. Application of high molecular weight partially imidized thermoplastic PI films in adhesive and composite matrix applications could be highly beneficial because the fluidity of the film will be high during the initial phase of processing. During this stage the highly fluid polyimide will provide good wetting of surrounding surfaces and in the latter part imidization will be completed to form a highly consolidated structure. Relationship between degree of imidization and viscoelastic properties has been investigated by Yi-Kun et al. for a thermosetting polyimide based on PMDA/ODA without any molecular weight control [12]. According to their results the behavior of PI films changes from viscoelastic liquid-like to viscoelastic solid-like as the degree of imidization increases. In their research the polyimide in question was thermosetting and have combination of rigid monomers, therefore the viscoelastic behavioral changes occurred at elevated temperatures (about 400 C). For thermoplastic polyimides with molecular weight control and semi-rigid monomer combination a similar behavior is expected but at relatively lower temperature and thus requires further investigation. In order to design a two step processing cycle, exact temperatures at which these behavioral changes takes place needs to be determined. 1.1. Selection of monomers In order to obtain PIs which could exhibit enhanced melt flow at processing temperatures the use of semi-rigid or flexible monomer units in the backbone is required. The monomers selected for this study were 3,3 0 ,4,4 0 -oxydiphthalic anhydride (ODPA) and 3,4 0 -oxydianiline (3,4 0 -ODA), which were anticipated to impart flexibility to the polyimide through their ether linking groups. Furthermore, in case of 3,4 0 -ODA the presence of meta-linkages increases the flexibility of the chains [13]. However

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M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

the polyimide based on ODPA/3,4 0 -ODA is amorphous in nature and is susceptible to solvent attacks. Therefore it was deemed necessary to include a semicrystalline PI in this study as semi crystalline PIs have better solvent resistance. The presence of crystallinity can also lead to a partial retention of properties much above the glass transition temperature, thus effectively increasing the high temperature resistance substantially [14]. Two commonly available anhydrides, pyromellitic dianhydride (PMDA) and 3, 3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride (BPDA) are well known for imparting crystallinity to PIs. PMDA has a more rigid structure comprising of one phenylene group as compared to BPDA having two. Therefore BPDA was selected as the monomer for the second PI investigated in this study. 1.2. Selection of molecular weight Theoretical average molecular weights (TMW) for both PIs precursors (PAA solution) were calculated for a combination of stoichiometric offsets in favor of 3,4 0 -ODA. The results are plotted in Fig. 1. From the figure it can be seen that TMW of PAA solution of both PIs increased significantly as the stoichiometric offset decreased below 2%. Difficulties in wetting of adherend surface or impregnation of fiber bundles can be experienced with such high molecular weights. Therefore a stoichiometric offset of 2% was selected for this study so that a desired molecular weight of around 24 kg mole1 could be achieved. It is important to note that above-mentioned estimate of TMW can differ signif-

100000 PI A PI B

TMW / g.mole

-1

80000

60000 Critical offset value

40000

2. Experimental

20000

0 0

icantly from the actual molecular weight of products of immidization of PAA. Along with the thermal imidization of films cast from PAA the occurrence of side reactions also take place. As a result of these side reactions the destruction of amide groups followed by resysnthesis takes place. The first step leads to a significant lowering of the PI molecular weight during the initial stages of thermal cyclization. The second step, i.e., resynthesis process, takes place at temperatures above 200 C. The molecular weight recovery process depends on chemical structure of PAA and imidization conditions. The molecular weight of samples imidized below 200 C will have significantly lower molecular weights as compared to molecular weight calculated theoretically. Therefore PI films partially imidized below 210 C were not tested for properties determination. Hou et al. [15] have reported that a significant difference may exist in theoretical and experimentally measured molecular weights of PAA precursors due to the presence of lower molecular weight (LMW) species and because of a larger polydispersity. However it must also be kept in mind that PAA precursor is hydrolytically unstable and this can result in degradation and chain scission even under low humid conditions. While carrying out experimental measurements using GPC, the dynamic chemistry of PAA may not provide accurate results. Keeping this in view, experimental measurement of molecular weights was not carried out. Since PAA associate through intermolecular hydrogen bonding of amide and acid groups along the chains, there is a chance that such an interaction might cause the LMW species to interact and to behave more like relatively longer molecules. The viscosity of PAA solution increases with increasing molecular weight and therefore the solution is required to be prepared at relatively low solid concentration in solvent. This is considered to be disadvantageous from the standpoint that more solvent might result in higher film shrinkage and formation of voids. Hence viscosity considerations were also considered while selecting the stoichiometric offset of 2%.

1

2

3

4

5

Stoichiometric Offset in favor of 3,4'-ODA / % Fig. 1. TMW versus percent stoichiometric offset of precursors (PAA solution) of PI A and PI B.

2.1. Materials Starting monomers BPDA, ODPA, and 3,4 0 ODA were supplied by Shanghai research institute of synthetic resins. Mono-functional end-capper

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

phthalic anhydride (PA) was supplied by Shijiazhang Bailong Chemical Co. Ltd. All monomers were dried in a vacuum oven at 170 C for 8–10 h prior to use. 1-Methyl-2-pyrrolidinone (NMP) was supplied by Tianjin Chemical Reagent Factory and was used as received. 2.2. Synthesis The first polyimide (ODPA/3,4 0 -ODA/PA) was synthesized as per procedure described elsewhere [16]. The monomer concentration was calculated using the Carother’s equation with a target TMW of 24 kg mole1 and is shown in Table 1. PAA solution with 30% solid concentration wt/wt was prepared by mixing the monomers in required quantity in NMP. The monomers were allowed to react at room temperature in NMP for long duration (16 h). The PAA solution was cast into films (thickness 0.02 and 0.05 mm) on glass plates which were thermally imidized in a far-infrared radiation (FIR) oven at different temperatures. The second polyimide (BPDA/3,4 0 -ODA/PA) was also synthesized using a similar procedure as described above for first polyimide. The two polyimides were coded as PI A and PI B as shown in Table 1 along with their repeat unit chemical structure and theoretical molecular weight (TMW).

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2.3. Characterization FTIR was utilized to track the imidization process by determining the functional groups in the PI oligomers and in partial/fully imidized films. The analysis was carried out with a Nexus-470 FTIR spectrometer (Nicolet). Thin film samples (thickness 0.02 mm) were utilized for this analysis. DMTA was utilized to analyze PI films, imidized at different temperatures, to determine storage and loss modulus values, as well as tan d. The instrument used was a Rheometric series TMA analyzer. For 0.05 mm thick PI films, the extension-mode deformation was used at a frequency of 1 Hz and a heating rate of 5 C/min under nitrogen. Tensile testing of PI films was carried out in accordance to ASTM standard D 882. The sample size used was 120 · 10 · 0.05 mm. The test was carried on a SAN 100 kN tensile testing machine equipped with a PC to acquire test data. Five samples were tested for each test condition and average of these five samples is presented as the final result for that particular test condition. 2.4. Determination of the imidization degree In this study the imidization degree was measured using FTIR absorbance spectra on the basis

Table 1 Molar ratios, TMW (PAA precursor) and chemical structure of polyimides A and B Code

Monomers (molar ratios)

PI A

ODPA (0.98)

3,4 0 -ODA (1.0)

Type and TMW PA (0.04)

Chemical structure

Thermoplastic; amorphous; 24 kg mole1

O

O

O

O

N

N

O

O

O

O

N

N

n

PI B

BPDA (0.98)

3,4 0 -ODA (1.0)

PA (0.04)

Thermoplastic; semi-crystalline; 24 kg mole1

O

O

O n

Table 2 Coding of samples according to imidization temperature Polyimide

PI A PI B

Imidization temperature/C 180

200

250

300

330

360

380

A1 B1

A2 B2

A3 B3

A4 B4

A5 B5

A6 B6

A7 B7

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M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

Table 3 Infrared absorption bands of imides and related compounds Absorption band/cm1 Aromatic amines

Amic acids

Standard

1780 1720 1380 725 2900–3300 1710 1660 1550 1500

Origin C@O asymmetric stretch C@O symmetric stretch C–N stretch C@O bending COOH and NH2 C@O(COOH) C@O(CONH) C–NH p-substituted benzene

of the Lambert–Beer Law and absorbance superposition [17]. According to Lambert–Beer Law the absorbance of the absorbent component is directly proportional to its concentration. Therefore absorbance of the film varies with the content of imide groups. During cyclodehydration of PAA, the imide absorption bands grow in intensity. Several of the absorption bands used for quantitative and qualitative analysis of PI and PAA are shown in Table 3. The absorption peak at 1500 cm1, corresponding to the C–C stretching of p-substituted benzene, was selected as an internal standard as it does not change during imidization process. Therefore, the degree of imidization could be indirectly characterized by the ratio of the absorbance at 1775 or 1380 cm1 to that at 1500 cm1. The absorbance can be calculated from transmittance in FTIR spectra, the method and formula were described in detail in previous papers [12].

PI film will contain residual uncyclized amic-acid structures, which are hydrolytically unstable and ultimately cause chain degradation. Therefore it is necessary to get rid of solvent by subjecting the films to low temperatures for prolong durations. A lot of heat is generated within polymer films causing the evaporation of solvent, even when the oven temperature is below the boiling temperature of NMP (202 C). For the processing of thick films (0.05 mm), it is also necessary that a controlled slow heating program is used. In case of a rapid heating process a hard skin is formed due to quick evaporation of the solvent from the surface, whereas the interior of the film still contains large amounts of solvents. As the temperature is further increased this dissolved solvent from the interior of the skin comes from beneath and causes massive bubbling at the film surface. A step wise heating schedule was adopted for imidization of the films for both PIs as shown in Fig. 2. Film samples were imidized up to 180, 200, 250, 300, 330, 360 and 380 C to determine the degree of imidization and related properties. Samples imidized at different temperatures were coded accordingly and are listed in Table 2. Films were heated for five hours at various temperatures below 100C. Within these five hours it was observed that films become tack free and solidify indicating large amount of solvent evaporation. Moreover, PI B films had a translucent appearance as compared to PI A films which were transparent. This translucency is attributed to semi-crystalline nature of PI B. The films obtained were smooth, creasable and free of bubbles and voids.

3. Results and discussion 400

3.1. Imidization process o

Temperature / C

The imidization process consists of cyclodehydration of PAA to form linear, stiff, ordered PI chains. As the rigid cyclic imide structure is formed during the imidization the chain mobility is decreased due to increase in stiffness. The resulting decrease in chain mobility hinders attainment of the intra-molecular conformation favoring the imidization reaction. Since the entropy is decreased, entities unfavorable to imide formation, such as solvent particles, remain rigidly fixed and the rate of imidization decreases [18]. Residual solvent molecules hinder attainment of imidization by forming intermolecular hydrogen bonds with the reactive groups. A rapidly imidized

A7, B7 A6, B6 A5, B5

350 300

A4, B4

250

A3, B3

200

A2, B2 A1, B1

150

Low temperature heating for 5 hours

100 50 0

1

2

3

4

5

6

7

8

9

10

Time / hours Fig. 2. Imidization schedule for PI A and PI B films imidized at different temperatures.

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

3.2. Degree of imidization FTIR analysis of PI A and PI B films imidized at 180, 200, 250, 300, 330, 360 and 380 C were carried out. Figs. 3 and 4 show the FTIR spectra of PI A and PI B films, respectively. For the purpose of clarity all spectra are not shown and only spectra of films imidized at 180, 200, 250 and 360 C are shown. Markedly significant changes can be observed in FTIR spectra from 180 to 250 C, whereas from 250 to 380 C changes in FTIR spectra were not very significant. This means that 250 C is the temperature up to which most of the imidization takes place. It has already been mentioned above, that during cyclode3300 -COOH

A6

1775 1500 1380 C-N C=O Std

A3

A2

A1

3500

3000

2500

2000

1500

1000

-1

Wavenumbers / cm

Fig. 3. FTIR spectra of partially imidized PI A films.

3300 -COOH

1500 1775 Std 1380 C=O C-N

B7

B3 B2

B1

3500

3000

2500

2000

1500

1000

-1

Wavenumbers / cm

Fig. 4. FTIR spectra of partially imidized PI B films.

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hydration process the chain stiffness increases as a result of formation of rigid cyclic imide groups. The decrease in imidization rate at higher temperatures is attributed to decrease in chain mobility, densification and configurational changes coming into effect as a result of increasing chain stiffness. From the FTIR spectra of films imidized at 180 C it can be seen that a broad peak appears at 3300 cm1 which corresponds to the absorption of COOH and NH2 present in the PAA. The presence of this peak indicates that amide groups are still present in the film. However, appearance of peaks at 1775, 1720, 725, and 1380 cm1 confirm the presence of imide groups thus indicating that imidization process has already started. In the FTIR spectra of film imidized at 200 C the peak at 3300 cm1 has disappeared whereas the peaks at 1775, 1720, 1381, and 720 cm1 have grown in intensity. This observation leads to the conclusion that rapid imidization takes place in this temperature range. It was observed that significant imidization takes place before temperature approaches the boiling point of NMP (202 C). It is necessary that large amount of solvent must be evaporated before reaching the temperature window of 180–210 C, and this can be achieved by exposing the films to lower temperatures (100–150 C) for longer durations. In the FTIR spectra of films imidized at 250 C it was observed that intensity of peaks corresponding to imide groups grew further thus indicating a progress in imidization process. This trend was also visible in FTIR spectra of films cured at 300, 330 and 360 C, though with increasing temperature the growth in intensity was of lower magnitude. 3.3. Rate of imidization Using Figs. 3 and 4, the ratio of absorbance at 1775 and 1380 cm1 to that at 1500 cm1 were calculated to obtain C@O/C@C and C–N/C@C ratios, respectively. The ratios measured were plotted against imidization temperatures and is shown in Fig. 5. It was observed that rate of imidization of PI A is faster than that of PI B. This higher rate of imidization can be attributed to the difference in the monomer structure and reactivity for these PIs. Monomer reactivity is important in controlling equilibrium to favor formation of PAA. High molecular weight PAA is obtained when the electron affinity of the dianhydride and the basicity of the diamine are both high [19]. A higher molecular weight PAA will imidize at a slower rate because

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

0.8 2.5

0.7

2.0

0.5 1.5

0.4 0.3

1.0 C-N/C=C PI A C-N/C=C PI B C=O/C=C PI A C=O/C=C PI B

0.2 0.1 0.0

50

100

150

200

250

300

350

C-N / C=C

C=O / C=C

0.6

0.5 0.0 400

Imidization temperature /ºC

Fig. 5. Comparison of imidization process of PI A and PI B films as measured from absorbences in FTIR analysis.

longer stiff chains will be immobilized more easily as compared to short length chains. The immobility of stiff chains is considered to be an unfavorable factor towards cyclodehydration of PAA. The aromatic tetracarboxylic acid anhydrides are highly electrophilic acylating agents toward amines. The enhanced electrophilicity results from strong electron-withdrawing effects exerted by the orthoplacement of the carbonyl groups. In case of PI B the dianhydride used is BPDA, which has a higher electron affinity as compared to ODPA which is used in PI A. ODPA has an electron donating either bridging group. The ether bridging group increases the electron density of the anhydride carbonyl carbon by delocalization of electrons through p-orbitals. BPDA does not have any bridging group between the two phthalic anhydride units and thus have a higher electron affinity as compared to ODPA. It is considered that difference in electron affinity of ODPA and BPDA will result in differences in molecular weight distribution in PAAs prepared from each monomer in conjunction with 3,4 0 -ODA. In case of PI A the number of LMW species will be higher as compared to PI B. 3.4. Post imidization effects or ageing in PI films In the FTIR spectra of PI A films imidized at 360 and 380C, peaks at 883(the deformation vibration of the isolated hydrogen atom in aromatic ring) and 1250 cm1 (the stretching vibration of the aromatic ether) become weak, whereas a broad absorption appears at 3400 cm1 that corresponds to the stretching vibration of the phenolic hydroxyl. This indicates occurrences of chain degradation pheno-

menon like aromatic ring dehydrogenation, aromatic ether splitting, and aromatic ring hydroxylation at higher temperature. Due to less rigidity in its backbone as compared to PI B, PI A is affected at a relatively lower temperature. 3.5. Mechanical properties It has been reported earlier that amorphous PI films exhibit a more prominent yield behavior as compared to semi-crystalline PIs before fracture when subjected to tensile stress [20]. The reason for yield behavior has been attributed to the presence of entangled macro-molecular chains which ravel and separates when subjected to tensile loads. In semi-crystalline PIs where the macro-molecules are stiff and are in ordered arrangement, the yield behavior under tensile load is less significant. Because of their ordered molecular arrangements in semi-crystalline PIs, the inter-molecular interactions are also very strong and contribute to high modulus of PIs. Figs. 6–9 show the tensile breaking strength, tensile modulus, tensile elongation at break (EB) and tensile breaking energy for films of PI A and B as a function of film imidization temperature, respectively. Films of PI A exhibited a larger EB and tensile breaking energy as compared to PI B films. The ether linkages in the ODPA and meta linkages in 3,4 0 -ODA are believed to impart this flexibility in PI A films. On the other hand BPDA because of its stiff molecular structure makes the PI B films less flexible but more strong. The highest EB and tensile breaking energy was observed in PI A films imidized at 300 C where as for PI B the highest values were observed in films

Tensile breaking strength / MPa

1850

PI A PI B

130 120 110 100 90 80 240

260

280

300

320

340

360

380

400

o

Imidization temperature / C Fig. 6. Tensile breaking strength versus imidization temperature.

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

2.5

2.0

1.5

1.0 240

260

PI A PI B

-3

Tensile breaking energy / MJ . m

Tensile modulus / GPa

60

PI A PI B

3.0

280

300

320

340

360

380

50 40 30 20 10 0

400

240

o

Imidization temperature / C

300

320

340

360

380

400

Fig. 9. Tensile breaking energy versus imidization temperature.

PI A PI B

imparts better mechanical properties to the PI. The increase in tensile strength at temperatures above 360C is very less thus indicating that imidization process is not advancing. This information is useful while designing cure cycle for adhesive resin and composite matrix resin applications.

30

3.6. Viscoelastic behavior

20

10

240

260

280

300

320

340

360

380

400

o

Imidization temperature / C Fig. 8. Tensile elongation at break versus imidization temperature.

imidized at 330 C. As mentioned earlier that PI B has a lower imidization rate therefore it will experience configurational changes and densification due to molecular ordering at a latter stage as compared to PI A. The molecular ordering in PIs is believed to make films stronger but less flexible. For PI A the films imidized at 360 and 380 C had almost same value of tensile stress but film imidized at 380 C had very less EB. This glassy behavior at high temperature is due to thermo-oxidative degradation of PI A as discussed in Section 3.4. The modulus for both PI A and PI B increases with imidization temperature, which indicates an increase in configurational changes and densification due to molecular ordering. The molecular ordering leads to better inter-molecular interactions and thus

Viscoelastic behavior of PI films is analyzed by carrying out loss tangent (tan d) measurements via DMTA. Tan d measurements were carried out for PI A and PI B films imidized at 250, 300, 330, 360 and 380 C. The measured values of tan d for PI A and PI B films imidized at different temperatures are shown in Figs. 10 and 11, respectively.

A4 1

Tan δ

Elongation at break / %

280

o

40

0

260

Imidization temperature / C

Fig. 7. Tensile modulus versus imidization temperature.

50

1851

A5 A6 A7

0.1

0.01

0

50

100

150

200

250

300

350

Temperature / oC Fig. 10. Tan d versus imidization temperature PI A.

400

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

1

B3 B4

B5 B6 B7

Tan δ

β-transitions

0.1

0.01 0

50

100

150

200

250

300

350

400

Temperature / oC Fig. 11. Tan d versus imidization temperature PI B.

According to Eq. (1), tan d is the ratio of the loss modulus (E00 ) and storage modulus (E 0 ) for a test sample deformed under frequency f, at time t and temperature T E00 ðf ; t; T Þ . ð1Þ tan dðf ; t; T Þ ¼ 0 E ðf ; t; T Þ When tan d > 1, the sample exhibits viscoelastic liquid-like behavior; and when tan d < 1, the sample exhibits viscoelastic solid-like behavior. It has been reported that polymer matrices with tan d values > 1 are relatively easy to thermoplastically mold into well consolidated composites; those with tan d < 1 were difficult to process [6]. The maximum tan d value for these samples was taken to represent Tg. During DMTA test the temperature of the partially imidized specimen is raised than the temperature at which film is imidized, and hence the samples are subjected to further imidization. The rate of imidization process is not constant and varies with imidization temperature. Near the glass transition temperature the rate of imidization is substantially high. Therefore during DMTA testing, advancement in imidization degree occurs which effects the properties of the sample to a certain degree. From Figs. 10 and 11 it can be seen that as imidization temperature is increased, the Tg peaks shift towards right, i.e., towards a higher temperature region. Similarly the magnitude of tan d also decreases with an increase in imidization temperature, indicating a decrease in fluidity of the films at the testing temperature [21]. As the imidization temperature increases, more amide groups cyclodehydrate to imide rings and molecular chains become stiffer. Therefore the elasticity of the films increases till a certain critical temperature, whereas

ageing occurs for films after this temperature. After this critical temperature densification and crosslinking among the molecular chains occurs resulting in reduction of flexibility of the films. For PI A films imidized below 330 C values of tan d greater than 1 are observed. This means that PI A films imidized below 330 C will exhibit viscoelastic liquid-like behavior whereas films imidized above this temperature will exhibit viscoelastic solid-like behavior at their respective Tgs. For PI B this viscoelastic solid-like behavior at Tg is observed for films cured at temperatures greater than 250 C. The higher value of Tg and lower value of tan d of PI B films is attributed to its stiff molecular structure. For PI B b-transitions are observed at 130– 140 C. These b-transitions are also reported in Kapton, which is a product of Dupont Company and is a semi-crystalline PI based on PMDA/ODA monomer combination and is available in form of films cast from PAA [22]. Sub-glass transition or b-transition peaks has been assigned to electric dipole orientation polarization resulting from the co-operative molecular motion of the residual reactive groups (C@O) and absorbed water in the film [23]. With increase in imidization temperature the magnitude of b-transition peaks is observed to decrease, which could possibly be an outcome of reduced amount of residual reactive groups. 3.7. Thermoplastic response Figs. 12 and 13 show the storage modulus (E 0 ) of PI A and B films, respectively, as a function of

1E9 A3

E' / Pa

1852

A4 A5 A6 A7

1E8

1E7

50

100

150

200

250

300

350

o

Temperature / C Fig. 12. Storage modulus of partially imidized films versus testing temperature PI A.

M.B. Saeed, M.-S. Zhan / European Polymer Journal 42 (2006) 1844–1854

E' / Pa

1E9

B3 B4 B5 B6 B7

1E8

1E7

50

100

150

200

250

300

350

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nation of benzene rings, make the PI films brittle and susceptible to catastrophic failure without elongation before rupture. (c) For both, PI A and PI B, the tensile modulus increases with increase in imidization temperature. The EB and tensile breaking energy are maximum for PI A films imidized at 300 C, where as for PI B films, the maximum values are recorded for films imidized at 330 C. (d) PI A films imidized below 330 C exhibits a viscoelastic liquid-like behavior at Tg, whereas PI B films imidized below 250 C exhibits such a behavior at Tg.

o

Temperature / C Fig. 13. Storage modulus of partially imidized films versus testing temperature PI B. 0

testing temperature. For both PIs the value of E at room temperature increases as the imidization temperature is increased. With increase in testing temperature the E 0 value drops and as the temperature approaches Tg this drop becomes more steep. This steep drop is more pronounced in PI A films as compared to PI B. This relative larger steep in drop of E 0 values of PI A indicates a better thermoplastic response as compared to PI B. Due to its flexible molecular structure, PI A exhibits greater thermoplastic fluidity and enhanced flow characteristics at temperatures approaching Tg. 4. Conclusion In this study, we have discussed the effects of monomer structure, monomer reactivity, PI molecular weight and degree of imidization on the mechanical properties and viscoelastic behavior of thermoplastic polyimide films cast from PAA. Properties of two different PIs are investigated and compared. (a) PI A imidizes at a faster rate as compared to PI B. The difference in imidization rate is attributed to differences in molecular structure and reactivity. (b) Most of the imidization process takes place in the temperature up to 250 C. The process continues at a slower pace till about 350 C in PI A and up to 380 C in PI B, after which molecular structural changes such as aromatic ring dehydrogenation, aromatic ether splitting, aromatic ring hydroxylation and combi-

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